1. Field of the Invention
The present invention relates to a fixing device, an image forming apparatus using the fixing device, and a temperature control method for the fixing device.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a copier, a printer, or a facsimile, generally uses a heat roller fixing device (including a heat belt fixing device) to fix toner attached to paper onto the paper using heat and pressure. Such a fixing device consumes a large amount of electrical power during operation to heat the paper quickly passing therethrough and compensate for heat loss due to contact with the paper. The power consumption for fixing is increased in a high-speed image forming apparatus as the number of paper passing through the fixing device per unit time increases. At the same time, however, a maximum power consumption specified for the image forming apparatus depends on a power supply situation, and tends not to provide sufficient electrical power under environmental and use-paper constraints particularly like those in effect in Japan or in North America. When the power supply is insufficient, the temperature of the fixing roller included in the fixing device cannot be maintained at a set temperature and declines during paper feed. In order to correct this tendency, the fixing device in the typical high-speed image forming apparatus consumes the maximum allowable amount of electrical power. Also, in a recent mainstream configuration in which a heater of the fixing device is turned off in a standby mode to save energy, the fixing roller needs to be rapidly heated to the set temperature, which also adds to overall electrical power requirements.
An additional problem is that the heater is started using electrical power including the electrical power saved by stopping other operations performed in the image forming apparatus. Consequently, there is a large difference between the heater power consumption while starting the heater and the heater power consumption during paper feed, which again adds to overall electrical power requirements.
Various attempts have been made to conserve power. Thus, for example, one example fixing device reduces heater power consumption to less than the rated power consumption by turning the heater on and off at short but regular intervals.
Another example fixing device activates a plurality of heaters by controlling the duty cycles of the heaters so as to reduce its power consumption for fixing the toner or for achieving a uniform temperature for the fixing roller. In this case, the fixing device is configured to reduce power consumption only when the plurality of heaters is activated.
However, in practical usage, allowable electrical power for the fixing device varies according to the mode of operation, and as such the heaters are unable to respond to such varying statuses simply by controlling their duty cycle. Therefore, such a fixing device is not useful in an image forming apparatus whose productivity is determined by the fixing ability maintained according to the electrical power.
In a typical fixing device including a heat roller having a center heater and an end heater, when a temperature of an end portion of a fixing belt heated by the end heater exceeds a set temperature thereof and a temperature of a center portion of the fixing belt heated by the center heater remains below a set temperature thereof, the end heater may be turned off while the center heater may be fully activated. In this case, however, even with the heater wattage at full activation for the center heater, less than the allowable electrical power for the fixing device is being used. Also, heat transfer through the heat roller from the end portion to the center portion stops by turning off the end heater, and therefore the temperature of the center portion may not be raised to the proper temperature.
By contrast, when the heater wattage for full activation of the center heater is equal to the allowable electrical power for the fixing device, the center heater cannot be efficiently activated in a mode in which the allowable electrical power for the fixing device is deliberately reduced as an energy-saving measure.